Corrosion resistant alloys

ABSTRACT

A non-austenitic (i.e., ferritic) corrosion resistant alloy useful in sea water environment has the following broad composition range: Chromium 16%-28%; Cobalt 4.5%-7.25%; Nickel 0%-6.5%; Silicon 1%-2.0%; Vanadium 0%-2.25%; Molybdenum 0%-4.25%; Carbon 0%-0.03%; Manganese 0%-0.1%; Iron - the balance; all parts by weight percent. A narrower composition range of these alloys is as follows: Chromium 16.5%-27%; Cobalt 4.75%-7.0%; Silicon 1.0%-2.0%; Molybdenum 0%-4.0%; Carbon 0%-0.01%; Manganese 0%-0.05%; Vanadium 0%-2.0%; Nickel 0%-6.0%; the balance iron; all parts by weight percent.

United States atent [191 Mendelson et al.

[451 Jan. 7, 1975 1 CORROSION RESISTANT ALLOYS [75] Inventors: Ralph A. Mendelson, Westminster;

Karl P. Staudhammer, Gardena; Roberto Valencia, Jr., Long Beach, all of Calif.

[73] Assignee: The United States of America as represented by the Secretary of the Interior, Washington, DC.

[22] Filed: Dec. 6, 1971 [21] Appl. No.: 205,358

Related US. Application Data [63] Continuation-impart of Ser, Nos. 103,818, Jan. 4, 1971, abandoned, and Ser. No. 113,074, Feb. 5, 1971, abandoned, and Ser. No. 119,933, March 1, 1971, abandoned, and Ser. No. 137,078, April 23, 1971, abandoned, and Ser. No. 137,006, April 23, 1971, abandoned.

[52] US. Cl. 75/126 C, 75/126 H, 75/128 B,

75/128 N, 75/126 Q [51] Int. Cl C22c 39/14 1 slt st g sly. 1 312 3 [56] References Cited UNITED STATES PATENTS 7/1921 Armstrong 75/126 H 4/1952 Kirkby 75/126 H Primary Examiner-Hyland Bizot Attorney, Agent, or FirmW. Krawitz [57] ABSTRACT A non-austenitic (i.e., ferritic) corrosion resistant alloy useful in sea water environment has the following broad composition range: Chromium 16%28%; Cobalt 4.5%7.25%; Nickel 0%6.5%; Silicon 1%-2.0%; Vanadium 0%-2.25%; Molybdenum O%4.25%; Carbon 0%0.03%; Manganese 0%-0.1%; lron the balance; all parts by weight percent.

A narrower composition range of these alloys is as follows: Chromium l6.5%27%; Cobalt 4.75%-7.07z; Silicon 1.0%2.0%; Molybdenum O%4.0%; Carbon 0%0.01%; Manganese 0%-0.05%; Vanadium 0%2.0%; Nickel 0%6.0%; the balance iron; all parts by weight percent.

4 Claims, N0 Drawings CORROSION RESISTANT ALLOYS This application is a continuation-in-part of Patent Application Ser. Nos. 103,818; 113,074; 119,933; 137,078; and 137,006 all now abandoned, filed on Jan. 4,l97l;Feb.5,l97l;Mar.1,1971;Apr.23,1971;and Apr. 23, 1971 respectively.

BACKGROUND OF THE INVENTION The invention herein described was made under a contract with the United States Department of the lnterior.

This invention relates to a new and improved alloy composition and more specifically to an alloy having corrosion-resistant properties, particularly in a sea water environment.

The use of metallic alloys in a corrosive sea water distillation facility requires that they have good corrosion resistance, good thermal conductivity, suitable tensile, yield and burst strengths, good fabrication properties and be inexpensive.

Copper-nickel alloys currently employed for heat exchanger pipes in desalination plants are typically 90-10 copper-nickel and 70-30 copper-nickel. However, the corrosion rate for these two alloys are 50 and 8 mils per year respectively (at 300F), and their ultimate tensile strengths are 44 X 10 and 55 X 10 psi respectively.

Furthermore, since both copper and nickel tend to be I in a relatively short supply, alloys formed from these two materials are fairly expensive.

Iron based alloys, particularly stainless steels, are less expensive than copper-nickel alloys and can be tailored to provide some of the desired properties such as corrosion resistance, thermal conductivity, tensile and burst strengths. However, all of these properties cannot be realized in any of the known conventional stainless steels. V

As an example, stainless steels require carbon to obtain good tensile and burst strengths, but the presence of carbon in significant quantities results in high corrosion rates when the steel is used in elevated temperature sea water. On the other hand, the use of carbon in very low amounts, or its entire omission, from a stainless steel will generally improve its corrosion resistance in sea water; however, the tensile and burst strengths may be lowered to unacceptable levels. Consequently,

stainless steels only provide a part of the essential char- It would be desirable to obtain an alloy having a corrosion rate in elevated temperature sea water of less than about 13 mils per year, a yield strength about three times that of the copper'nickel alloy and containing a large concentration of iron to reduce the cost of the materials employed in the alloy.

Due to its tendency to stress corrosion cracking, it is necessary when employing steel compositions in sea water environment, to suppress the formation or retention of austenite steel which has a face-centered lattice structure and promotes the formation of a ferrite structure which is body-centered cubic in structure.

Also, it is necessary to balance the individual components of an alloy in such a manner as to provide suitable tensile, burst and conductivity properties.

Furthermore, it is necessary that the alloy composition retain its ferritic phase, body-centered, cubic structure at room temperature after undergoing temperature cycling and not the face-centered lattice structure of austenite steel.

In view of the foregoing, it is an object of this invention to provide easily fabricable iron based alloy compositions having improved corrosion resistance to sea water while still retaining suitable physical properties of thermal conductivity, tensile and burst strengths.

Other objects of the invention will become apparent from the description to follow.

THE INVENTION According to the invention, the broad limit of the non-austenitic (i.e., ferrite) corrosion resistant alloy composition is as follows: Chromium l6%-28%; Cobalt 4.5%-7.25%; Nickel 0%6.5%; Silicon 1%2.0%; Vanadium 0%-2.25%; Molybdenum 0%-4.25%; Carbon 0%-0.03%; Manganese 0%-0.1%; Iron the balance; all parts by weight percent.

A narrower composition range of these alloys is as follows: Chromium 16.5%-27%; Cobalt 4.75%-7.0%; Silicon 1.0%2.0%; Molybdenum 0%4.0%; Carbon 0%0.01%; Manganese 0%0.05%; Vanadium 0%-2.0%; Nickel 0%-6.0%; the balance iron; all parts by weight percent.

Typically, the carbon content is about 0.005%, while the manganese content is less than about 0.1%.

To a limited extent, tungsten can be substituted in part for molybdenum, while columbium and/or tantalum may be substituted in whole or in part for vanadium.

The alloys of the invention are ferritic phase, having a body-centered cubic lattice structure at room temperature.

There are many surprising features concerning the effects of the individual alloying ingredients. For example, the effect in stainless steels of carbon and manganese is to increase tensile properties with increasing concentration of these two ingredients. The alloys of this invention by contrast have high tensile strengths comparable to the 300 stainless steel series when the carbon and manganese are present at very low levels, or are preferably omitted entirely.

Also, in stainless steels, the use of carbon, nickel, and manganese promote the formation of austenite. In the present alloys, however, the ferritic phase is produced instead due to the combined balancing effect of the other components of the alloy.

Furthermore, the use of silicon instead of carbon has the effect of enhancing the tensile properties of the present alloys without forming the undesirable austenitic structure.

The overall effect, therefore, is to provide an inexpensive iron based alloy having the high tensile and thermal conductivity properties of stainless steels, but which resists sea water corrosion to a very marked degree.

Specific alloys within the above range having the following approximate (i1%) compositions are shown in Table l as follows:

TABLE 1 ALLOY COBALT CHRO- SILICON MOLYBDE- VANA- NICKEL IRON MIUM NUM DIUM SAMPLE A 6.9 20.5 1.7 0.3 0 Balance B 4.8 16.7 2.0 4.0 0 0 do.

C 5.9 26.7 1.0 0 0 0 do.

Sample E which follows is another non-austenitic (i.e., ferritic phase) alloy composition of the invention having similar suitable properties for use as pipes, heat exchangers, etc., in a sea water environment and has the following approximate (11%) composition: Chromium 20.0%; Vanadium 2.0%; Silicon 1.2%; Nickel 6.0%; and the balance iron, all parts by weight percent. Cobalt is present only to the extent of itsbeing an impurity and usually present in nickel. Sample E also has a body-centered cubic lattice structure.

The corrosion resistance of the alloys was determined by first preparing rolled strips of the alloy from are cast ingots. The rolling was carried out by heating the ingot to about l650-l 675C under Argon and then rolling in a standard cold rolling mill. The process was repeated until the rolled strip was of the desired thickness. Uncoated probes for electrical-resistance measurements were prepared from the strip and were tested in 300F (150 psi) natural sea water using a Magna Corrosometer. In the testing conditions used, the sea water was saturated with oxygen.

A sample of a 90-10 copper-nickel alloy was also tested in the same test cell. The testing was carried out for various test periods and from the electrical resistance measurement of the strips, the corrosion rate of the copper-nickel alloy was calculated to be about 40-50 mils per year. On corresponding tests, samples A-E of the present invention had the following corrosion rates as shown in Table 2.

Furthermore, there was no accompanying pitting reaction. I

By comparison, 304 stainless steel has poor corrosion resistance to sea water due to its high carbon content (0.08%); this is in the order of at least l0 mils/year accompanied by severe pitting.

Tensile measurements were determined from miniature tensile bars which were machined from the as- Lolled strip of the alloy. The results of the miniature tensile tests were verified by employing identical samples of 304 stainless steel as a compairson. A standard cross head (ASTM E-8, 1967) was employed at a speed of 0.050 inches per minute to pull the tensile specimens. The following results shown in Table 3 were obtained with a 304 stainless steel as a comparison. ,8

It will be obvious from Table 3 that the tensile and yield properties of the present alloys are quite comparable to the 304 stainless steel.

.The thermal conductivity of the alloys A-E varied .from about 8-1 1.3 BTU/hr/ft /ft/F 320% and is of the same magnitude as stainless steels.

Accordingly, it will be seen that the alloy composition of the present invention represents a significant improvement over copper-nickel alloys employed in desalination plants not only from the standpoint of corrosion resistance, but also in tensile properties and in cost. Furthermore, the alloy of the present invention can be readily fabricated by conventional techniques without any deleterious results on end use. This permits it to be employed for a variety of uses such as in heat exchangers, boilers, pipes, etc. Since its tensile properties are significantly greater than those of coppernickel alloys, tubing thickness and hence weight can be significantly reduced. This in turn means that the structural requirements of a desalination plant employing the alloy of the present invention can be markedly reduced. Thus, a much less expensive plant design and capital investment is required.

What is claimed is:

l. A ferritic phase alloy of body-centered cubic lattice structure consisting of Cobalt 6.9%; Chromium 20.5%; Silicon 1.7%; Mo-

lybdenum 0.3%; the balance iron; all parts by weight percent.

2. A ferritic phase alloy of body-centered cubic lattice structure consisting of Cobalt 4.8%; Chromium 16.7%; Silicon 2.0%; Mo-

lybdenum 4.0%; the balance iron; all parts by weight percent.

3. A ferritic phase alloy of body-centered cubic lattice structure consisting of Cobalt 5.9%; Chromium 26.7%; Silicon 1.0%; the

balance iron; all parts by weight percent.

4. A ferritic phase alloy of body-centered cubic lattice structure consisting of Cobalt 6.0%; Chromium 18.5%; Silicon 1.0%; Mo-

lybdenum 0.5%; Nickel 6.0%; the balance iron; all parts by weight percent.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 3,859,080

DATED January 7, 1975 INVENTOR(S) Ralph A. Mendelson, Karl P, Staudhammer, Roberto Valencia Jr. It rs certrfred that error appears In the above-rdentrfred patent and that sard Letters Pa'tent Q are hereby corrected as shown beiow:

Column 4, line 17, delete 8.

Column 4, following line 17, insert the following table:

TABLE 3 m ELONGATION IN ALLOY YIELD STRENGTH (psi) ULTIMATE TENSILE (psi) ONE INCH SAMPLE After Heat After Heat After Heat As Rolled Treatment* As Rolled Treatment* Treatment A 130 X 56 X 10 133 X 10 75.2 X 10 22 B 130 X 10 74.4 X 10 158 X 103 93.2 X 10 c 120 X 10 53 X 10 121 X 10 71 X 10 25 D 152 X 10 34 X 10 174 X-10 114.7 X 10 41 E 149 X 10 72.7 X 10 154 X 10 91.6 X 10 20 304 Stain- 39.8 X 10 91.6 X 10 less Steel -10 Nickel 44 X 10 15 X 10 50 Copper *To produce minimum hardness or Signed and Scaled this fourteenth Day of October 1975 [SEAL] Attest:

RUTH c. MASON c. MARSHALL DANN Arresting Officer Commissioner 0] Patents and Trademarks 

1. A FERRITIC PHASE ALLOY OF BODY-CENTERED CUBIC LATTICE STRUCTURE CONSISTING OF COBALT 6.9%; CHROMIUM 20.5%; SILICON 1.7%; MOLYBDENUM 0.3%; THE BALANCE IRON; ALL PARTS BY WEIGHT PERCENT.
 2. A ferritic phase alloy of body-centered cubic lattice structure consisting of Cobalt 4.8%; Chromium 16.7%; Silicon 2.0%; Molybdenum 4.0%; the balance iron; all parts by weight percent.
 3. A ferritic phase alloy of body-centered cubic lattice structure consisting of Cobalt 5.9%; Chromium 26.7%; Silicon 1.0%; the balance iron; all parts by weight percent.
 4. A ferritic phase alloy of body-centered cubic lattice structure consisting of Cobalt 6.0%; Chromium 18.5%; Silicon 1.0%; Molybdenum 0.5%; Nickel 6.0%; the balance iron; all parts by weight percent. 